This invention relates to a current limiting device for an alternating-current electric power system and, more particularly, relates to a current limiting device that achieves current-limiting action on the first loop of current following fault inception by rapidly diverting fault current through a low impedance path to ground.
A current limiting device may be thought of as a device which is capable of limiting the peak current allowed to pass therethrough into a negligible impedance fault at its load terminal to a value substantially below the peak of the prospective current of the system at the fault. In referring to the prospective current of the system at a given point in the circuit, applicant is using this terminology in the sense defined in the American National Standard Definitions for Power Switchgear, Publication ANSI C37-100-1972, pages 8 and 9, published in 1972 by the Institute of Electrical and Electronic Engineers, New York, N.Y.
Most efforts to achieve a practical current limiting device for power systems rated at more than a few thousand volts have approached the current limiting problem in a very difficult, head-on manner, attempting to achieve current limitation by rapid insertion of an impedance, either resistive or inductive, into the power system to limit the first current peak. Such approaches have been subject to some fundamental difficulties, such as one or more of the following: prohibitive initial cost, inability to quickly transfer current into the impedance being inserted, unacceptable continuous losses, excessive complexity and resultant low reliability, and inadequate voltage and current handling capabilities. Examples of these approaches are disclosed in a special report (EPRI EL-276-SR) prepared by the Electric Power Research Institute, Palo Alto, Calif., entitled Symposium Proceedings-New Concepts in Fault Current Limiters and Power Circuit Breakers. This report is dated April, 1977 and contains papers presented at a symposium in Buffalo, New York, on Sept. 28-30, 1976.